1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to a metal-ion battery using a hybrid ion electrolyte with two types of cations.
2. Description of the Related Art
Transition metal hexacyanoferrates (TMHCFs) with large interstitial spaces have been investigated as cathode materials for rechargeable lithium-ion batteries [1, 2], sodium-ion batteries [3, 4], and potassium-ion batteries [5]. With an aqueous electrolyte containing the proper alkali-ions or ammonium-ions, copper and nickel hexacyanoferrates ((Cu,Ni)—HCFs) exhibited a very good cycling life that retained an 83% capacity after 40,000 cycles at a charge/discharge current of 17 C (1 C=150 milliamp hours per gram (mAh/g) [6-8]. However, the materials demonstrated low capacities and energy densities because (1) just one sodium-ion can be inserted/extracted into/from each Cu-HCF or Ni—HCF formula and (2) these TMHCFs electrodes must be operated below 1.23 V due to water electrochemical window. The electrochemical window of a substance is the voltage range between which the substance is neither oxidized nor reduced. This range is important for the efficiency of an electrode, and once out of this range, water becomes electrolyzed, spoiling the electrical energy intended for another electrochemical reaction. To correct these shortcomings, manganese hexacyanoferrate (Mn—HCF) and iron hexacyanoferrate (Fe—HCF) were used as cathode materials in a non-aqueous electrolyte [9, 10]. Assembled with sodium-metal anode, Mn—HCF and Fe—HCF electrodes cycled between 2.0V and 4.2 V and delivered capacities of about 150 mAh/g.
However, the as-prepared TMHCF, which consists of an alkali metal such as Li, Na and K inside the Prussian Blue framework in the discharged state, exhibits rapid capacity decay during repeated sodium insertion/extraction in a non-aqueous electrolyte like carbonate-based organic electrolytes. For example, Na2MnFe(CN)6 shows a capacity retention less than 75% after 100 cycles under a modest current density, and Na2Fe(CN)6 prepared via hydrothermal also exhibits a 20% capacity loss in 120 cycles. Such poor capacity retention hinders commercial applications of TMHCF-based rechargeable batteries. Although Berlin Green, which has an empty framework of FeFe(CN)6, has demonstrated a 1000× cycle life, it is impractical for large scale applications because it requires a sodium metal anode as a sodium source, which is a serious safety issue for batteries. On the other hand, the substitution of high-spin iron or manganese with nickel results in stable capacity retention, but the reversible capacity is less than 80 mAh/g, which is too low for practical applications.
It would be advantageous if an electrolyte containing hybrid conductive ions, such as alkali and alkaline earth ions, could be used in a TMHCF-based metal-ion battery to enable ultra-long cycle lifetimes.    [1] V. D. Neff, Some performance characteristics of a Prussian Blue battery, Journal of Electrochemical Society, 132 (1985) 1382-1384.    [2] N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, T. Yamagishi, Lithium intercalation behavior into iron cyanide complex as positive electrode of lithium secondary battery, Journal of Power Sources, 79 (1999) 215-219.    [3] Y. Lu, L. Wang, J. Cheng, J. B. Goodenough, Prussian blue: a new framework for sodium batteries, Chemistry Communication, 48 (2012) 6544-6546.    [4] L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, J. B. Goodenough, A superior low-cost cathode for a Na-ion battery, Angew. Chem. Int. Ed., 52 (2013) 1964-1967.    [5] A. Eftekhari, Potassium secondary cell based on Prussian blue cathode, J. Power Sources, 126 (2004) 221-228.    [6] C. D. Wessells, R. A. Huggins, Y. Cui, Copper hexacyanoferrate battery electrodes with long cycle life and high power, Nature Communication, 2 (2011) 550.    [7] C. D. Wessells, S. V. Peddada, R. A. Huggins, Y. Cui, Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries. Nano Letter, 11 (2011) 5421-5425.    [8] C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, Y. Cui, The effect of insertion species on nanostructured open framework hexacyanoferrate battery electrode, J. Electrochem. Soc., 159 (2012) A98-A103.    [9] T. Matsuda, M. Takachi, Y. Moritomo, A sodium manganese ferrocyanide thin film for Na-ion batteries, Chemical Communications, DOI: 10.1039/C3CC38839E.    [10] S.-H. Yu, M. Shokouhimehr, T. Hyeon, Y.-E. Sung, Iron hexacyanoferrate nanoparticles as cathode materials for lithium and sodium rechargeable batteries, ECS Electrochemistry Letters, 2 (2013) A39-A41.    [11] D. Asakura, M. Okubo, Y. Mizuno, T. Kudo, H. Zhou, K. Ikedo, T. Mizokawa, A. Okazawa, N. Kojima, Fabrication of a cyanide-bridged coordination polymer electrode for enhanced electrochemical ion storage ability, J. Phys. Chem. C, 116 (2012) 8364-8369.